Generally, the elements of such a control panel, i.e., a cover plate and keys, each include a plastic substrate and a light-blocking coating formed on the outer surfaces thereof. The coating on the key is treated by, for example, laser beam so as to form an operation indicating sign for indicating the operation effected by operation of the key. In order to make the color tone of the entire outer surface of the control panel uniform, the coatings formed on the surfaces of the cover plate and keys are formed from the same material. It often occurs, however, that since these elements are generally formed separately and then assembled together, the color tone of the coatings of these elements slightly differs from each other.
Further, when a control panel is assembled in such a manner, the keys are prone to being set in incorrect positions in the cover plate.
Furthermore, if it is attempted to first assemble the plastic substrates of the elements and then to form coatings on the surfaces of the substrates, it is difficult to uniformly form the coatings on all of the surfaces, because, for example, the keys generally have a front surface parallel to the surface of the cover plate and side surfaces generally normal to the front surface, thereby causing slight leakage from the side surfaces, on which the coating is not sufficiently formed, when the key is actuated and the light behind the cover plate is turned on.